In the manufacture of electrical coils for motors, insulation is placed over a coil and is held in place with a binding tape. The coil is immersed in a solventless resin which is under pressure. The coil is then removed from the solventless resin and the resin is cured. The purpose of the resin is to hold the insulation in place and to fill the voids. Voids permit moisture to enter the coil which can short-circuit the coil and burn it out.
A persistent problem in this process has been that the resin drains out of the insulation and through the tape before the resin can be cured. Considerable efforts have been made to find a tape which would permit the resin to flow into the insulation but would not permit the resin to drain out. A Mylar (polyethylene terephthalate) tape has been used but that tape, even when butt-lapped, did not permit the resin to flow into the insulation well which resulted in voids and poor bonding of the insulation to the copper coil. Also, that tape was expensive and had to be removed after the resin was cured because it was not thermally stable, which was an additional operation and which left the coil looking untidy.
Butt-lapped woven glass tape has also been used but it retained the resin poorly and the coil had to be impregnated several times.